Generating XUV or X-Ray radiation with sufficient photon fluxes for the applications named above on the basis of an interaction of laser light with a target like electron bunches or atomic clusters requires on one hand high average and preferably even higher peak powers and on the other hand high photon flux densities, as the efficiency of the XUV/X-Ray generation process is low (depending on the target, e. g. 10−6). High photon flux densities can be provided with laser light generated by active amplification or by coherent addition in a passive resonator device (see e. g. C. E. Clayton et al., “Nuclear Instruments and Methods in Physics Research A” vol. 355, 1995, p. 121-129./R. J. Jones et al., “Phys. Rev. Lett.” vol. 94, 2005, p. 193201-1 to 193201-4, or US 2006/0268949 A1).
Coherent addition of laser light in a passive resonator device (resonator device without amplifying medium, enhancement resonator) is described e. g. in U.S. Pat. No. 6,038,055 or by Y. Vidne et al. in “Optics Letters” vol. 28, 2003, p. 2396-2398. Laser light is coupled into an optical resonator device comprising at least two resonator mirrors in standing wave geometry, at least three resonator mirrors which are arranged with a ring geometry or at least four resonator mirrors in a bow tie ring configuration. The resonator device forms a light path having a resonator length, which is adjusted such that input laser light coupled into the optical resonator device interferes constructively with the intra-resonator laser light to form high power intra-resonator laser light.
The conventional generation of laser light using an enhancement resonator has limitations in terms of power and photon flux density. If conventional resonator mirrors having a reflectivity of 99.9% corresponding to a reflectivity loss of 1000 ppm are used, an arrangement of e. g. four resonator mirrors yields a complete loss of at least 4000 ppm and a corresponding limit of the theoretically possible enhancement. If tuned dielectric resonator mirrors (as described e. g. by H. R. Bilger et al., “Appl. Opt.” vol. 33, 1994, p. 7390-7396) having a reflectivity loss of only 30 ppm to 100 ppm are used, the intra-resonator power enhancement can be increased up to e. g. 1000. However, there is still a limitation as to the stored laser light photon flux density on the mirrors. Optical nonlinearities in mirror coatings and impurities on mirror surfaces may yield additional losses, the later e. g. by locally increased temperatures leading to destruction of the mirror surface in the form of microscopic burning spots. Furthermore, impurities, burning spots and particles on the mirror surface induce scattering losses and thermal lensing effects yield average power dependent wavefront distortions. Therefore, the conventional laser light generation employing an enhancement resonator is limited to an average power below 80 kW. As an example, I. Pupeza et al. (“Optics Letters” vol. 35, p. 2052-2054 (2010)) report on an average power of 72 kW. However, for some applications, the generation of XUV/X-Ray or THz radiation based on an interaction of laser pulses, e. g. with bunches of relativistic electrons or atomic clusters, may require a peak photon flux density in the range of 1015 W/cm2 to 1016 W/cm2; thus demanding intra-resonator values of an average power of several 100 kW up to two MW, an 1/e2 focal radius of the photon flux density below 25 μm and a pulse duration below 250 fs.
The interaction of the stored laser light with a target can be provided at at least one location within the enhancement resonator. For obtaining high and finely adjustable photon flux densities, the target is supplied in the vicinity of a focal position between a pair of spherical mirrors being arranged with a distance L (optical length at the central frequency of the laser light between the intersections of the laser light path with the two mirror surfaces) and each having a radius of curvature R. In a conventional enhancement resonator the 1/e2 beam radius of the photon flux density on the mirrors is about 500 μm, resulting in a high photon flux density on the mirror surface yielding nonlinear response and damages above a critical photon flux density threshold.
PCT/EP2009/008278 (unpublished on the priority date of the present specification) discloses a method of generating high power laser light, wherein the resonator mirrors are irradiated by the at least one laser light pulse circulating in the enhancement resonator with oblique incidence. Due to the oblique incidence, the area irradiated by the laser beam on the resonator mirror surfaces is increased, thus the photon flux density on the mirror surfaces is decreased. However, this technique may have disadvantages as providing the oblique incidence may require a complex mirror and mirror positioning design.
Another general disadvantage of conventional techniques using the generation of coherent XUV/X-Ray radiation within the enhancement resonator may occur with regard to the output coupling. Coherent XUV/X-Ray radiation may be coupled out of the resonator through a small opening within one of the resonator mirrors. Although the small opening may has a typical diameter of only 100 μm, it may essentially affect the function of the mirror as a result of diffraction, scattering and reduction of the reflecting surface. With a 1/e2 beam radius of the photon flux density on the mirrors of about 500 μm, the small opening may represent a source of significant losses to the intra-resonator laser light.